A digital cellular radio telephone system that is proposed for use in the United States employs TDMA as a channel access method. For this proposed system data is transmitted in frames, each of which comprises three or six user time slots. Each user time slot is uniquely identified by a reference feature, or sequence of symbols referred to as a preamble to which is typically appended a data portion.
A transmission standard (IS-54) proposed for use with the U.S. digital cellular radio telephone system uses a TDMA frame structure. Each TDMA frame has a duration of 40 msec and comprises six time slots, each having a duration of 6.67 msec. Each frame is comprised of 1944 bits, or 972 symbols, where each symbol is comprised of two bits, for a gross transmission rate of 48.6 Kbits/sec. Each time slot contains a 28 bit preamble which is a carefully chosen bit sequence used for synchronization. The remainder of the time slot contains digitized voice data along with several bits necessary for guard time, in-band signalling (i.e., slow associated control channel or SACCH) and a digital verification color code (DVCC). Each time slot is generally assigned to one user.
In such a system, and prior to detecting the transmitted data portion, it is necessary to identify and locate the beginning of the assigned time slot. This is accomplished by identifying the start of the TDMA frame and then identifying the start of the desired time slot within the frame. The identification of the start of the frame and time slot is referred to as synchronization.
The TDMA frame structure is such that the identification of a particular time slot marks the beginning of a particular user's data frame. Hence the term frame/slot synchronization is used herein as opposed to differentiating the method into separate frame and time slot synchronization tasks.
After reception of the serial data stream a demodulator produces a baseband signal that comprises a stream of (I,Q) samples representing the in-phase (I) and quadrature (Q) components of the received signal. These received samples are used to establish frame and slot synchronization.
One known synchronization method involves locating a peak of a correlation between a known preamble and the corresponding received sequence in a particular time slot. The underlying concept of this method is referred to herein as a correlation peak search. As was stated, each time slot in the TDMA frame is identified by a unique reference feature or preamble. The preamble symbols are chosen such that their autocorrelation sequence resembles white noise, with cross-correlation between preambles being very low. On an error-free channel, the high autocorrelation and low cross-correlation between the preambles provides for discrimination between the time slots. Thus an autocorrelation sequence similar to white noise provides for locating the precise beginning of the preamble.
One technique for obtaining frame/slot synchronization for a TDMA system that is representative of the method employed in the CEPT/GSM (European) digital cellular system is discussed by G. D'Aria and V. Zingarelli in "Design and Performance of Synchronization Techniques and Viterbi Adaptive Equalizers for Narrowband TDMA Mobile Radio", Proceedings of Third Nordic Seminar on Digital Land Mobile Radio Communication, Copenhagen, Denmark, 9/13-15/88. The synchronization method described therein is based on the previously mentioned correlation peak search.
However, there are several significant differences between the CEPT/GSM system and the U.S. digital cellular system. These include the type of modulation, filtering, and data rate, all of which require modifications to the correlation peak search synchronization method. These differences between systems manifest themselves as requiring a different method of frame/slot timing acquisition and a different method for subsequent maintenance of this timing.
It is thus an object of the invention to provide method and apparatus for achieving and maintaining frame acquisition in the U.S. digital cellular radio system.
It is a further object of the invention to provide a simplified, relative to the prior art, method and apparatus for achieving and maintaining frame acquisition in the U.S. digital cellular radio system.
It is another object of the invention to provide method and apparatus for achieving and maintaining frame acquisition in the U.S. digital cellular radio system that performs a correlation between a reference sequence of symbols and a corresponding received sequence forming a portion of a particular time slot, the correlation peaks thus obtained being processed to lower the probability of an occurrence of a false alarm and/or a false lock.